reqParams.js

function required(params) {
  return function (req, res, cb) {
    params.every(function (elem) { return req.body.hasOwnProperty(elem); })
      && cb()
      || res.send(400);
  };
};

/**
 * Sample route
 */
app.get("/route", required(["user_string"]), function(req, res) {
  var respObj = {}
      user_string = req.body.user_string;

  res.writeHead(200, {'content-type': 'application/json'});
  res.end();
});